1. Field of the Invention
The present invention relates to an improvement of a recording head to be employed in thermal recording or a liquid ejection (ex. inkjet) recording.
2. Description of the Related Art
FIG. 29 is a plan view showing a portion of a heat generating resistor portion of a thick film thermal head as the conventional recording head disclosed in Japanese Unexamined Patent Publication (Kokai) No. Hei 01-150556, for example. In FIG. 29, 1 denotes a strip form common electrode, 2 denotes a plurality of common electrode leads extending from one edge of the strip form common electrode 1 in a comb-like fashion, 3 denotes a plurality of individual electrode leads respectively having one end positioned between two common electrode leads, and 4 denotes a strip form resistor formed by applying a resistor paste, such as that composed of ruthenium oxide and a glass component, over the common electrode leads 2 and the individual electrode leads 3 and drying and sintering the same. Each of the individual heat generating resistors 6 consists of two heat generating resistors 61 and 62 disposed between the common electrode leads 2 and the individual electrode leads 3. The interval between the leads is uniform at L. Also, the individual electrode leads 3 are connected to elements to perform switching according to printing information, at a not shown position. It should be noted that a protection layer and so forth which cover the heat generating resistors 6 to provide wear resistance and anti-oxidation purpose are not shown.
Next, description will be given of the operation of the conventional thermal head. By selectively driving one of the individual electrode leads 3, one thermal resistor unit 6 constituted by the heat generating resistors 61 and 62 is heated. The thermal resistor unit 6 is pressed onto a thermal paper as a recording paper (not shown) to cause color development by heating of the thermal resistor 6. The temperature distribution of the thermal resistor 6 is such that it has two elliptical high temperature portions with the highest temperature at the central portions HL and HR of the heat generating resistors 61 and 62, as shown in FIG. 30A. FIG. 30B is a section taken along line A-B of the plan view of FIG. 30A and shows that the cross-section of the strip form resistor 4 has a barrel-shaped configuration. This configuration results from formation of the strip form resistor 4 by application of the resistor paste.
The resistance value of the thermal resistor unit 6 is the value resulting from the parallel combination of the heat generating resistors 61 and 62. However, the resistance value may fluctuate in each of the heat generating resistors to a certain extent. A lower resistance value results in a greater current value with respect to the same voltage and results in a greater color development area. For performing high quality printing, it is necessary for the color development areas of respective heat generating resistors to be uniform. Therefore, the heat generating resistors have to be formed to have uniform resistance values.
As a method for unification of the resistance values of the heat generating resistors, there is a pulse-trimming method as disclosed in U.S. Pat. No. 4,782,202. The proposed method permits manufacturing under a standard with the average resistance of respective heat generating resistors being within a range of .+-.3% and non-uniformity of the individual heat generating resistors being within a range of .+-.15% (standard deviation within .+-.2%).
Hereinafter, a brief explanation will be given of the pulse trimming method.
FIG. 31 shows variation of the resistance value when a pulse having a voltage higher than that of normal use is applied to the heat generating resistor. In FIG. 31, when a pulse having a voltage greater than V0 is applied, the resistance is lowered. In order to adjust the resistance to a desired value Rx, a pulse having a voltage Vx may be applied. However, the pulse voltage is not necessarily applied as a single pulse. It is possible to sequentially apply a pulse with a lower voltage a plurality of times.
Namely, a sequential pulse is applied, and the effect of each pulse is accumulated as thermal energy. FIG. 32 shows a relationship between a number of pulses and the resistance value in the case where the voltage is applied by dividing it into a plurality of pulses. The case where relatively low voltage pulses are applied is shown by a solid line and the case where relatively high voltage pulses are applied is shown by broken line.
As shown in FIG. 32, while application of low voltage pulses may result in a long period for adjustment of the resistance, it may be advantageous for permitting delicate adjustment of the resistance.
Since the conventional thermal head is constructed as set forth above, uniformity of the resistance of the heat generating resistor 6 can be achieved. However, one problem still remains which cannot be solved by the method set forth above. Namely, what is unified by the pulse trimming is the resistance value of the thermal resistor unit 6, specifically the parallel combination of the heat generating resistors 61 and 62. In other words, there may still be a deviation of the resistance values between two heat generating resistors 61 and 62. As a result, a problem of inclination of the configuration of the color development dot due to a difference of the resistance values of the heat generating resistors 61 and 62 remains which limits improvement of the uniformity of the color development by the pulse trimming method. Due to the high voltage pulse which is applied, the lowest resistance portion of each of the heat generating resistor 61 and 62 produced by pulse trimming method may flucture with respect to specified value resistance. This may be influenced by particle distribution of the resistor material component and insulation material component in the paste of the ruthenium oxide as the resistor material. Accordingly, it becomes impossible to make the heat distribution of the thermal resistor 6 uniform which causes a problem of non-uniformity of the configuration and size of the color development dots.
As improvements for the configuration of the color development dots in the thick film thermal head, there are known prior arts disclosed in Japanese Examined Utility Model Publications (Kokoku) Nos. Hei 5-18144, Hei 5-181145 and Hei 5-181146. Even in such cases, it is not possible to unify the heat distribution when resistance trimming for the heat generating resistor is performed. Also, Japanese Unexamined Patent Publication No. Hei 2-243360 discloses to provide a higher resistance for one of the common electrode lead or the individual electrode lead for improving color development distribution of the thick film thermal head. However, a difficulty is encountered in unification of high resistance in production.